In order to integrate electrical signals, it is known practice to use an analog passive low-pass filter which, in the simplest case, consists of a resistor/capacitor combination (RC element). Such integrators are used, for example, in electronic circuit-breakers in order to determine the currents flowing through the circuit-breaker since current transformers with Rogowski coils, which generate a voltage that is proportional to the temporal derivative of the current, are usually used as current sensors. An analog passive low-pass filter has the advantage that it largely suppresses (short-circuits) radio-frequency interference, which simplifies subsequent signal processing, in particular. In this case, the integration capacitor with a relatively large capacitance is directly connected, for example, to the two inputs (input pins) of the subsequent integrated circuit (for example an operational amplifier). The disadvantage of such a low-pass filter is that the phase response deviates greatly from that of an ideal integrator, to be precise also when the base frequency of the low-pass filter is considerably below the frequency range of interest, as a result of which the amplitude response has only a relatively small error.
In contrast, the phase error is large and can also be corrected only with difficulty by means of digital signal processing on account of its non-linearity. However, in principle, the phase error can be eliminated by means of active integration, for example by connecting the capacitor to the negative feedback path (the feedback loop) of an operational amplifier in a simple design, which results, in practice, in an ideal amplitude and phase response in the frequency range of interest. However, if a single-ended input (all signals are related to ground) is used, one side of the capacitor is still at the input signal, but the other side is not at ground, where it cannot discharge any radio-frequency interference either, but at the output of the operational amplifier. In this manner, radio-frequency interference can pass into the operational amplifier, which can result in unmanageable errors. This problem also exists with a symmetrical (differential) input circuit in which the solution is to short-circuit the radio-frequency interference by way of a capacitor between the two inputs (input pins).